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Qin, C., Li, S., Jiang, G., Cao, J., Guo, Y., Li, J., Zhang, B., and Han, S. (2017). "Preparation of flower-like ZnO nanoparticles in a cellulose hydrogel microreactor," BioRes. 12(2), 3182-3191.


Flower-like zinc oxide (ZnO) nanoparticles were synthesized with sodium hydroxide and zinc acetate in a cellulose hydrogel microreactor (prepared by the inversion method). The samples were characterized by scanning electron microscopy, EDX, x-ray diffractometry, ultraviolet-visible diffuse reflection spectroscopy, and nitrogen adsorption-desorption. The results indicated that ZnO grows in a flower-like shape in the pores of the cellulose hydrogel. The pure hexagonal wurtzite structures have uniform diameters in the range of 10 nm to 30 nm, surface areas of 39.18 m2/g, and pore volumes of 0.2109 cm3/g. This study also investigated the photocatalytic properties. The nanoparticles have a band gap of 3.23 eV and a 95.2% efficiency for the ultraviolet degradation of rhodamine B over 3 h at room temperature.

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Preparation of Flower-like ZnO Nanoparticles in a Cellulose Hydrogel Microreactor

Chengcheng Qin,a Shujun Li,a Guiquan Jiang,c Jun Cao,b,* Yuliang Guo,a Jiangwei Li,a Bin Zhang,a and Shiyan Han a,b,*

Flower-like zinc oxide (ZnO) nanoparticles were synthesized with sodium hydroxide and zinc acetate in a cellulose hydrogel microreactor (prepared by the inversion method). The samples were characterized by scanning electron microscopy, EDX, x-ray diffractometry, ultraviolet-visible diffuse reflection spectroscopy, and nitrogen adsorption-desorption. The results indicated that ZnO grows in a flower-like shape in the pores of the cellulose hydrogel. The pure hexagonal wurtzite structures have uniform diameters in the range of 10 nm to 30 nm, surface areas of 39.18 m2/g, and pore volumes of 0.2109 cm3/g. This study also investigated the photocatalytic properties. The nanoparticles have a band gap of 3.23 eV and a 95.2% efficiency for the ultraviolet degradation of rhodamine B over 3 h at room temperature.

Keywords: Flower-like ZnO; Cellulose hydrogel microreactor; Photocatalytic; Nanoparticle

Contact information: a: Material Science and Engineering College and b: Postdoctoral Station of Mechanical Engineering, Northeast Forestry University, Harbin 150040, China; c: Forestry College of Beihua University, Jilin 132013, China; *Corresponding author:


Zinc oxide (ZnO) has been widely used in photocatalysts (Han et al. 2012), solar cells (Wu et al. 2010), transistors (Ong et al. 2007), piezoelectric transducers (Choi et al. 2010), and gas sensors (Spencer and Yarovsky 2010). The II-VI compound semiconductor has outstanding electrical and optical properties (Cho et al. 2009) as well as excellent chemical and thermal stabilities. Zinc oxide has a wide direct band gap at 3.37 eV and an exciton binding energy of 60 meV (Su et al. 2010). Due to its high photosensitivity, low cost (Zhu et al. 2012), and nontoxicity, ZnO is an important semiconductor photocatalyst. Composite materials with ZnO have better photocatalytic activity under visible light (Shaker-Agjekandy and Habibi-Yangjeh 2016; Shekofteh-Gohari and Habibi-Yangjeh 2016). Generally, the shape, size, and crystalline structure of semiconductors determine their chemical and physical properties. Zinc oxide materials have been prepared with different sizes and morphologies, such as spheres (Deng et al. 2008), flower-like shapes (Zhao et al. 2007), tubes (Li et al. 2008), cages, prismatic shapes, ellipsoidal shapes, hollow shells (Yu and Yu 2008), dumbbell shapes, nanowires (Ghoshal et al. 2008), nanorods (Cui et al. 2009), and nano-bundles (Muruganandham and Wu 2008). Various methods yield the large variety of shapes, sizes, and crystalline forms, as well as differing optical properties (Colón et al. 2008). These methods include hydrothermal synthesis (Hu et al. 2008), sol-gel processes (Rani et al. 2008), chemical deposition (Lokhande et al. 2009), chemical vapor deposition (Hu et al. 2012), and gas-phase reactions.

Hydrogels, which are three-dimensional networks of hydrophilic polymers, can absorb and retain a substantial amount of water. They are formed via hydrogen bonds, physical entanglement, chemical cross-linking, and ionic bonds. Due to hydrogels being hydrophilic, permeable, biocompatible, and having a low coefficient of friction, they have been widely used for drug delivery, water-absorbing resins, and cosmetics. Hydrogels prepared from natural polymers, especially polysaccharides, have promising applications as biomaterials because they are abundant, biodegradable, and non-toxic. Cellulose is an abundant, renewable polysaccharide raw material used in hydrogels. Chemical cross-linking, using water-soluble cellulose derivatives and bifunctional cross-linkers, is an elegant route to prepare hydrogels. Synthesizing cellulose-based hydrogels by radiation-induced polymerization is also convenient. Lastly, transparent cellulose hydrogels can be made by cellulose regeneration via changing the solution conditions (i.e., pH, temperature). These hydrogels (Isobe et al. 2013) are highly porous, with 50 nm to 100 nm pores and large surface areas of 300 m2/g to 400 m2/g (Chang et al. 2010).

For this study, hydrogels were synthesized with a “one-step” method from unsubstituted cellulose that was dissolved directly in a sodium hydroxide (NaOH)/urea aqueous solution by adjusting the pH and temperature. The cellulose hydrogels were then used as a microreactor for ZnO growth in the pore structure. The ZnO morphology, structure, and performance were characterized.



Bamboo fiber (98% cellulose) was prepared by slicing, steaming, cooking, and enzyme processing. It was purchased from Mingtong Bamboo Charcoal Products Co., Ltd. (Jinhua City, Zhejiang Province, China). Zinc acetate dihydrate (Zn(CH3COO)2·2H2O), NaOH, urea, anhydrous ethanol, tert-butyl alcohol, and rhodamine B were supplied by Aladdin Industrial Inc. (Shanghai, China). All chemicals were analytical-grade reagents and used without further purification. Deionized water was used throughout the experimentation.

Preparation of Cellulose Hydrogel and ZnO

Regenerated cellulose hydrogel was fabricated by the inversion method. When a 7:12:81 mass ratio solution of NaOH/urea/H2O (100 g) was cooled to -12 °C, 2.0 g of bamboo fiber was added and dispersed with vigorous stirring for 3 min (Cai et al. 2004; Zhou et al. 2004). The transparent cellulose solution (5 mL) was then added to a 10-mL sealed beaker and placed in a 50 °C water-bath for 24 h, yielding the cellulose hydrogel (Cai and Zhang 2006).

The hydrogel was transferred to a 100-mL beaker, immersed in 50 mL of water, and washed three times until the pH was 9. The surface water of the hydrogel was exchanged with ethanol via flushing. Subsequently, the hydrogel was immersed in a 100 mL of ethanol solution containing 0.1 wt.% zinc acetate for 48 h in a 40 °C water bath. The resulting ZnO precursor/cellulose hydrogel hybrid was washed with ethanol three times, then removing the water in precursor by absolute ethanol 72h and immerging tert-butanol 12 h of 3 times, at last freeze drying. The resulting ZnO precursor/cellulose aerogel hybrid was finally calcined in a muffle furnace in air at 500 C for 3 h to obtain the flower-like ZnO.


Scanning electron microscopy (SEM; QUATA200, FEI Company, Hillsboro Oregon, USA) of the cellulose hydrogel, ZnO precursor/cellulose hydrogel hybrid, and ZnO was performed. The purity and elemental analysis of the ZnO were obtained by EDX on the same SEM instrument. X-ray diffractometry (XRD; Rigaku D/max-2200, Tokyo, Japan) was performed with a D/max-rB diffractometer with 40 kV and 30 mA Cu Kα radiation in the 2θ range of 20° to 80°, at a scanning speed of 4°/min. Transmission electron microscopy (TEM) of the ZnO particle size distribution was performed with a H-7650 instrument (Hitachi, Tokyo, Japan). The surface areas were determined from nitrogen adsorption-desorption isotherms using an ASAP 2000 instrument (Coulter Omnisorp 100CX, Beckman Coulter Company, Norcross, GA, USA) and Brunauer-Emmett-Teller (BET) calculations. The UV-visible diffuse reflection spectra of ZnO were acquired on a TU-1901 UV-visible spectrophotometer (equipped with an integrating sphere) (Beijing Purkinje General Instrument Co., Ltd., Beijing, China), using BaSO4 as a reference.

Photocatalytic Activity

The photocatalytic activities of the ZnO nanocrystals were evaluated by monitoring the degradation of rhodamine B in an aqueous solution in a reactor described previously (Sun et al. 2009). The ZnO (10 mg) and 200 mL of 10 mg/L rhodamine B solution were placed in the reactor and mixed with an air diffuser, which was placed at the bottom of the reactor to uniformly disperse air into the solution at a flow rate of 0.5 m3/h. The solution was initially stirred for 30 min to reach absorption equilibrium and was then exposed to 300 W of UV light, with the strongest emission at 365 nm. The reactor was kept at room temperature via a water bath. To monitor the rhodamine B degradation, a sample was collected by centrifugation every 30 min and characterized with the UV-visible spectrophotometer set at the 550 nm rhodamine B absorption maximum. For a control experiment, the same reaction without catalyst was monitored under the same conditions.


SEM of Flower-like ZnO and its Formation Mechanism

Figures 1a, b, c, and d show the morphologies of the cellulose hydrogel, the ZnO precursor/cellulose hydrogel hybrid, ZnO, and ZnO at higher magnification, respectively. Notably, the cellulose hydrogel fibers were intertwined with each other forming a porous network structure (Liu et al. 2014). The ZnO was grown in the shape of flowers, and this morphology was maintained during roasting in a muffle furnace.

From the SEM images, as well as from previous reports (Zhang and Mu 2007; Kuriakose et al. 2013), the formation of flower-like ZnO structures involves three stages, which are shown in Fig. 2. Their growth strongly depends on the concentration of Zn2+ and OH ions, and it involves the following processes: (1) nucleation and growth of ZnO nanoparticles; (2) oriented attachment of the nanoparticles to form nanosheets; and (3) self-assembly of the nanosheets into three-dimensional flower-like structures. Here, ZnO nanostructures were synthesized from NaOH and zinc acetate in a 40 °C water bath, and the cellulose hydrogel was used as a microreactor. While the cellulose hydrogel was prepared, the NaOH/urea/H2O solution was used as the solvent system, and the hydrogel surface water was exchanged with ethanol. Consequently, NaOH, urea, and H2O were present in the hydrogel pores. After the hydrogel was immersed in the ethanol solution of zinc acetate, the NaOH and zinc acetate reacted as follows (Bian et al. 2011; Sun et al. 2012).

Zn2+ +2OH → Zn(OH)2 (1)

Zn(OH)2 → ZnO + H2O (2)

Zn(OH)2 + 2OH ↔ [Zn(OH)4]2- (3)

ZnO + H2O + 2OH ↔ [Zn(OH)4]2- (4)

Fig. 1. SEM of the ZnO samples: (a) cellulose hydrogel; (b) ZnO precursor/cellulose hydrogel hybrid; (c) ZnO (×2000); and (d) ZnO (×15000) magnification

Fig. 2. Formation mechanism of flower-like ZnO structures

Due to the water and the chemical bonding between the NaOH/urea/H2O and the cellulose hydrogel, the reaction proceeded slowly in the pores. In the presence of excess OH ions (a higher NaOH concentration in this case), [Zn(OH)4]2- ions were formed. The dehydration of [Zn(OH)4]2- led to the nucleation and growth of ZnO nanoparticles. Subsequently, the urea induced the oriented attachment of the ZnO nanoparticles to form ZnO nanosheets (Wu et al. 2005). Finally, to minimize the surface energy the nanosheets were self-assembled into three-dimensional flower-like structures. The uniform pore sizes in the cellulose hydrogel microreactor led to the uniform dimensions of the ZnO structures.

Analysis of XRD and EDX

The XRD patterns of the ZnO sample are shown in Fig. 3a. The high intensities of the diffraction peaks suggest that it was very crystalline (Zhao et al. 2013). All peaks matched well with the hexagonal wurtzite structure of ZnO (JCPDS card No. 36-1451). No noticeable impurities were detected; this indicated that the sample was pure ZnO.

Fig. 3. (a) XRD of the ZnO sample, (b) EDX of the ZnO sample and (c) UV-visible diffuse reflection spectroscopy of the ZnO sample

The purity and elemental analysis of the ZnO were obtained by EDX on the same SEM instrument. As indicated in Fig. 3b, no impurity was detected. The Au element was introduced by SEM (the sample was sprayed with Au ) and the C element was mainly due to CO2 in the air because the prepared ZnO did not show the presence of the C element according to XRD.

UV-Visible Diffuse Reflection Spectroscopy

The absorption of UV and visible light is an important photocatalytic property. As shown in Fig. 3c, the ZnO had a broad absorption band that extended from the ultraviolet to the visible region, with a maximum absorption at 384 nm. This intrinsic band-gap absorption was a result of the electron transitions from the valence to the conduction band. The absence of other bands confirmed the sample purity and indicated a uniform microstructure. The ZnO band gap energy Ebg was estimated to be 3.23 eV, according to Ebg= 1240/λ (eV) (where λ is the 384-nm absorption maximum), (Ebg– Band gap energy of semiconductor eV, λ-Irradiation wavelength threshold nm) (Zhao et al. 2013; Venkatachalam et al. 2007).

BET Analysis

Figure 4(a) reveals the nitrogen adsorption-desorption isotherms of the ZnO samples. The isotherms were type IV, with H3 hysteresis loops that are characteristic of porous materials. Thus, the samples had a mesoporous texture with a uniform interparticle distribution. The BET analysis indicated a surface area of 39.18 m2/g, whereas the pore volume, as determined by the Barrett-Joyner-Halenda method, was 0.2109 cm3/g, with an average pore size of 21.5 nm.

Fig. 4. (a) N2 adsorption-desorption of the ZnO sample, and (b) photocatalytic degradation curves of RhB over ZnO

Photocatalytic Degradation of Rhodamine B

As shown in Fig. 4b, the concentration of rhodamine B (RhB) did not change over 180 min in the absence of the ZnO photocatalyst, which indicated that the catalyst was essential for efficient photodegradation (Zhao et al. 2013). It was also clear that the RhB concentration changed remarkably during the pre-absorption with the UV light off, which may have been due to the large ZnO surface area for the adsorption of RhB (Zhou et al. 2012). When the UV light was switched on, the photocatalytic reaction was initiated, and the RhB concentration decreased greatly. After catalyzed 180 min, the degradation rate of rhodamine B by ZnO, with the addition of hydrogel and no hydrogel, was 95.2% and 45.2%, respectively. This good result can be attributed to the preparation of the ZnO sample using the hydrogel as a microreactor. As reported previously, photocatalysts with higher surface energies exhibit better photocatalytic performances. Moreover, the (0001) plane exhibits the highest surface energy for ZnO (Zhang et al. 2007; Xu et al. 2009). Thus, the flower-like ZnO nanosheets, which had large areas in the (0001) orientation, exhibited good photocatalytic activities.


1. With bamboo fiber as raw material, a cellulose hydrogel with uniform pore size was prepared by inverse method, and flower-like ZnO was prepared using the hydrogel as a micro reactor and zinc acetate as raw material.

2. The prepared ZnO using hydrogel as micro reactor exhibited a flower structure with uniform morphology according to SEM. The material was hexagonal wurtzite according to XRD, and it was pure ZnO by EDX. The band gap width was 3.23eV based on UV. The specific surface area and pore volume were 39.18 m2/g and 0.2109 cm3/g, respectively.

3. After the process of catalysis had taken place for 180 min, the degradation rate of rhodamine B by ZnO with the addition of hydrogel and without hydrogel was 95.2% and 45.2%, respectively. The results showed that the photocatalytic activity of ZnO using hydrogel as micro reactor was higher.


This work was supported by the Fundamental Research Funds for the Central Universities (2572014CB03), the Young Technology Innovation Talent Foundation of Harbin (2013RFQXJ035), the Young Science Foundation of Heilongjiang (QC2013C033), and the Key Projects of Science and Technology Department of Jilin Province (20140204006GX).


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Article submitted: October 23, 2016; Peer review completed: December 13, 2016; Revised version received: March 4, 2017; Accepted: March 5, 2017; Published: March 13, 2017.

DOI: 10.15376/biores.12.2.3182-3191